Presently, printed circuit boards are fabricated as part of a larger panel. Each printed circuit board can be configured in any shape, although most printed circuit boards in common use are made in rectangular shapes of standard sizes. When fabrication of a printed circuit board is complete, it is cut and separated from the larger panel, mostly by way of a machine cutting or routing process in which a channel is cut around the printed circuit board. In certain designs, the channel around the printed circuit board does not completely encircle the perimeter of the printed circuit board. Rather, tabs are left at several places around the perimeter of the printed circuit board to attach it to the larger panel until the board is singulated from the larger panel by breaking the tabs. Typically, metal planes in the printed circuit board do not extend to its edge where they would be cut by the routing process. In this way, no conductive metal is left exposed on the edges of the printed circuit board.
The existing methods of cutting printed circuit boards from larger panels are unsatisfactory for high density boards because the limited dimensional stability of the printed circuit boards does not allow registration of one high density pattern to the next on the larger panel. Cut lines made around the periphery of the printed circuit board further weaken the panel material, exasperating misregistration of one pattern to the next.
Electronic systems assembled onto conventional printed circuit boards rely upon thermal conduction from integrated circuits dissipating heat to the printed circuit board to remove some of the heat from the integrated circuits. For intermediate ranges of heat, up to about 2 watts per chip, conduction to the printed circuit board is sufficient to cool the integrated circuits without the need for bulky and expensive heat sinks. In high performance systems, however, as the density of the system and the percentage of the substrate covered by the integrated circuits increases, the thermal path to the printed circuit board is less efficient. At a point when the density of the system increases sufficiently, the printed circuit board is not effective as a heat sink for the integrated circuits. However, the need for effective thermal conduction from the integrated circuits to the substrate and therefrom to the ambient becomes more important as system density increases. Because of the evolution toward higher system density and larger integrated circuit coverage, means are needed for cooling the substrate in order to maintain the integrated circuits on the substrate at a safe operating temperature.
In addition to thermal conduction, high performance systems increasingly require low impedance power and ground voltage supplies to run the integrated circuits at high clock speeds. Typically, the AC impedances of power and ground supplies are lowered by the use of low impedance bypass capacitors connected to the power and ground planes. On conventional printed circuit boards, capacitors are connected to power and ground planes through vias which extend through some thickness of the board, increasing the impedance of this contact and degrading performance of the system. As switching speeds increase, the problem of making low impedance connections between bypass capacitors and the power and ground planes becomes more important.
It is, therefore, an object of the present invention to overcome the above problems and others by providing a printed circuit board having one or more printed circuit board layers having a conductive plane which extends to the edge of the printed circuit board but which is substantially, but not completely, covered by an insulating material. The edge of the conductive layer not covered by the insulating material can be on the edge of a tab which is utilized to couple the board to a disposable part of a larger panel that the board is formed from during fabrication. The exposed edge of the conductive layer becomes exposed upon breaking the tab and singulating the printed circuit board from the disposable part of the larger panel. In one embodiment, the tab terminates in a recess in the perimeter of the printed circuit board. In another embodiment, the tab extends outward from the perimeter of the printed circuit board.
The conductive plane can be formed from metal, such as a copper foil, that can serve the dual purpose of conducting heat away from electrical components disposed on one or both surfaces of the printed circuit board or printed circuit board layer and for providing power or ground to the electrical components. The tab which extends outward from the edge of the second embodiment of the printed circuit board can be coupled to a mechanical fixture and/or an electrical fixture to provide a path for the flow of heat from the printed circuit board to externally coupled mechanical fixtures and/or to provide electrical power to the electrically conducting layer of the printed circuit board. Still other objects will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.